Regardless Of Size, Animal Reflexes Are 'Remarkably Slow' (2024)

Whether responding to the sudden pounce of a predator or the loss of balance before a harmful fall, an animal’s survival may ultimately depend on how quickly it can react. So neural delays — from registering a stimulus all the way to the physical response — can be potentially costly.

It’s not entirely clear, though, how reflexes vary in relation to an animal’s size, but a team of researchers in Canada decided to find out.

"Animals as small as shrews and as large as elephants are built out of the same building blocks of nerve and muscle," says Max Donelan, a professor of Biomedical Physiology and Kinesiology at Simon Fraser University in Canada.

"We sought to understand how these building blocks are configured in different sized animals, and how this limits their performance," he says.

He and his colleague Heather More, who is also at Simon Fraser University, took a closer look at the several neural delays involved in animal reflexes.

Firstly, there is a delay involved in converting a stimulus to a nerve signal in the first place — for example, photons of light must be converted to optical signals, and sound waves must trigger messages in the auditory nerves.

There’s also a delay associated with conducting those signals along the length of each nerve, as well as something called a ‘synaptic delay’, which is the time required to send a signal, in the form of neurotransmitters, between synapses at nerve endings. Then there is a lag time involved in sending signals from the nerves to the muscles, to say nothing of delays involved in generating force within those muscles. Quite a lot happens in the blink of an eye, as it were.

Previously, Donelan and colleagues had found that the speed of nerve fiber conduction is nearly constant across the full size range of terrestrial animals. So, as nerve fibers get longer in larger animals, the more time it takes to transmit information. Nerve conduction ranges from around 1 millisecond in small animals to about 70 milliseconds in large animals such as elephants.

More, who is first author on both the previous and current studies, puts it this way, “to compare to engineered systems, it takes less time for an orbiting satellite to send a signal to earth than for an elephant's spinal cord to send a signal to its lower leg."

In the new study, More and Donelan wanted to see how all the delays influence the total reflex time in terrestrial animals of different sizes.

"Not surprisingly, we found that reflexes take a lot longer in large animals,” says More, explaining that they take around 17 times longer in the largest animals compared to the smallest.

However, the delays involved in sensing stimuli, and sending messages between synapses and between nerves and muscle are relatively quick, they explain. In other words, the time it takes for nerves to talk to one another or to talk to the muscles doesn’t significantly add to the problem.

Moreover, they found that larger animals are able to compensate for slow nerve conduction with a few strategies. For one thing, they tend to move more slowly.

“Remarkably, and in contradiction to our hypothesis, increases in total delay with increases in animal size are mostly offset by the longer movement durations of larger animals,” More and Donelan explain in their new paper, published in the Proceedings of the Royal Society B.

“Simply moving more slowly provides more time to respond to disturbances,” they explain. “Large terrestrial mammals appear to benefit from this strategy — elephants, for example, move almost 90% slower at their top speed than predicted based on smaller animals.”

Donelan adds that larger animals may also use prediction to think ahead about the consequences of their movements and adjust accordingly.

It turns out that it’s not all smooth running for small animals either. They too have to contend with sensorimotor delays and come up with work-arounds.

More explains that because the nerve fibers in small animals are so short, the time it takes for signals to run along those nerves isn’t the major issue for them. Instead, the time it takes for signals to jump across synapses is more time consuming in relation to their body size, than it is for big animals.

"This synaptic delay is one measure of the time to think,” says More. “So large animals have lots of time to think about how to respond to a disturbance, whereas as small animals don't."

"If a small animal puts its foot in a hole when sprinting, there is barely enough time for it to adjust its motion while the foot is on the ground,” she says.

Donelanadds that small animals most likely compensate for their limitations in a different way than large animals do.

"We suspect that small animals rely on pre-flexive control, where their bodies are built in such a way that they can reject disturbances like stepping in a hole without intervention from their nervous system,” he says.

In their paper, More and Donelan explain that this strategy employs innate biomechanics to quickly mitigate disturbances.

“For example, animals stabilize their motion using the passive dynamics of their moving body, the geometry of their legs, and the intrinsic properties of muscle.”

So it seems that whether an animal is tiny, enormous, or somewhere in between, sensorimotor delays represent a challenge for controlling muscle responses — especially when speed is of the essence.

"When running quickly, all animals are challenged by their lengthy response times which comprise nearly all of their available movement time,” says More.

“Even the fastest reflex for the control of running is remarkably slow."

Original Research:

More, HL and Donelan, JM (2018)Scaling of sensorimotor delays in terrestrial mammals.Proc. R. Soc. B 285: 20180613